DNS Zone Transfer Verification

Hi,

I've been searching around on how to verify/check if our Zone Transfer is working successfully or not but I can't seem to find any article to show just that. I only managed to find article on how to configure the DNS Express and Zone Transfer https://devcentral.f5.com/articles/dns-express-and-zone-transfers.

We already have an existing configuration for Zone transfer but we recently encounter issue with one of the "zone" that we recently added where the other end are getting an error "Zone Not Loaded by DNS Server" "The zone transfer was not executed.Refused".

Below are parts of the named configuration in F5. Transfer is already allowed.

options {
    listen-on port 53 {
        127.0.0.1;
        "zrd-acl-000-000";
        "zrd-acl-000-001";
        "zrd-acl-000-002";
        "zrd-acl-000-003";
        "zrd-acl-000-004";
        "zrd-acl-000-005";
        "zrd-acl-000-006";
    };
    listen-on-v6 port 53 {
        ::1;
    };
    recursion no;
    directory "/config/namedb";
    allow-transfer {
        localhost;
    };

Below is the zone file for the zone in question.

   zone "" {
        type master;
        file "db.external..";
        allow-update {
            localhost;
        };

From the DNS profile, the zone transfer is also enabled as well as DNS express though I cant seem to find the exact DNS Express module/option. Our current version is 11.6.2 HF1.

I have tried to do dnsxdump but based on the output there doesnt to be anything being transferred to DNS express.

[admin@name:Active:In Sync] ~  dnsxdump
DNS-Express DB Dump

-= Arena Allocator =-


-= Region Stats =-
memory: 7 objects (7 small/0 large), 336 bytes allocated (5 wasted) in 1 chunks, 0 cleanups, 0 in recyclebin 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0

-= DB Dump =-

-= DB Stats =-
RR Count: 0
Name Count: 0
RR Count by Type:

And LTM/GTM logs, they doesn't show anything regarding DNS Zone transfer. So I'm not sure what we have been missing.